The invention relates generally to systems, devices and methods for the compression, expansion, and/or storage of a gas, such as air or natural gas, and particularly to systems, devices and methods for optimizing the efficiency of such compression, expansion, and/or storage of air or natural gas, including optimizing the efficiency of controlling the temperature and/or water content of natural gas during such compression, expansion, and/or storage.
Compressed air energy storage systems (CAES) are a known type of system in limited use for storing energy in the form of compressed air. CAES systems may be used to store energy, in the form of compressed air, for example, when electricity demand is low and then to release the energy when demand is high. Such systems include at least one compressor that operates to compress air for storage; and at least one expander (such as an air turbine) that operates to extract power from expansion of the stored, compressed air. Known CAES-type systems for storing energy as compressed air may have a multi-stage compressor that may include intercoolers that cool air between stages of compression and/or after-coolers that cool air after compression. In such a system, however, the air may still achieve substantial temperatures during each stage of compression, prior to being cooled, which will introduce inefficiencies in the system. Thus, an improvement to known CAES type systems for compressing and/or expanding air is needed, and the applicants have appreciated the improvements needed to benefit gas storage using CAES type systems.
Natural gas providers are subject to fluctuating consumer demand for natural gas. For example, the demand for natural gas may be lower in warmer months and higher in cooler months. During periods of lower demand, natural gas in excess of demand may be burned at a flare, thus wasting this natural resource. During periods of higher demand, natural gas supply may lag behind demand. As such, an energy storage system capable of storing natural gas during periods of low demand for use during periods of high demand may improve overall operating costs and reliability for natural gas suppliers.
Current systems for storing natural gas include filtering the natural gas from a pipeline for dust or small particles, and then measuring the gas for quantity and quality. The natural gas is then either directly injected into a storage chamber, if the pipeline pressure is greater than the chamber pressure, or the natural gas is compressed with conventional compressors and then injected into the storage chamber. Such conventional compressors can include intercoolers and/or aftercoolers to disperse excess heat produced during the compression process.
Natural gas can include water, which may cause corrosion of natural gas distribution pipelines and/or which is in excess of regulatory requirements. As such, the excess water should be removed from the natural gas prior to distribution of the gas through the natural gas distribution pipelines. In some known systems, stored natural gas withdrawn from a storage chamber is expanded to pipeline pressure, processed to ensure pipeline gas quality, metered for quality and volumetric flow, and delivered to a pipeline system. The applicants have appreciated that a natural gas storage system may benefit from a compression and/or expansion system that has improved efficiencies.
Known storage chambers for storing natural gas include depleted natural gas reservoirs, and, to a much lesser extent, salt caverns. Additionally, some CAES systems utilize underground storage caverns to store compressed air. Such known devices and systems that utilize underground storage caverns may require that the storage cavern be maintained at some minimum pressure to prevent damage to and/or collapse of the storage system. For example, in salt cavern storage, one concern is preserving the structural integrity of the cavern. Each cavern may be required to be maintained above a minimum pressure, below a maximum pressure, and/or operated within a maximum high-pressure to low-pressure pressure range. Some guidelines (e.g., cavern operation regulations of the Federal Energy Regulatory Commission and the Texas Railroad Commission) are that the pressure range over which a cavern may be operated is dependent on the depth of the cavern. For example, some caverns have a maximum allowable operating pressure of 0.75 psi per foot of depth of the top of the cavern below the surface and a minimum allowable operation pressure of 0.2 psi per foot of depth of the top of the cavern below the surface. This results in a operational pressure range of 0.55 psi per foot of depth of the top of the cavern below the surface. Thus, a cavern that is 3,000 ft below the surface would have a maximum allowable pressure operation pressure of 2,250 psi, a minimum allowable operation pressure of 600 psi, and an operational pressure range of 1,650 psi. Thus, a minimum quantity of gas (at a given temperature) would need to be maintained in the cavern—removal of too much gas could lead to collapse of the cavern and/or lead to a violation of operation regulations. This requirement of retaining a certain pressure/quantity of stored gas in the cavern limits the effective working capacity of the storage structure. Other caverns may have a maximum allowable operating pressure less than 0.75 psi per foot of depth and/or a minimum allowable operation pressure greater than 0.2 psi per foot of depth and thus, further limiting the effective working capacity of the storage structure. The requirement to maintain a minimum gas pressure, and thus a minimum quantity of gas, in the storage cavern means that a significant quantity of stored gas may not be recovered from the storage cavern, and thus not available for use or sale. Thus, there is a need to improve the storage of compressed gas, and of natural gas in particular.